1. Field of the Invention
The present invention relates to radio frequency transmission and reception circuits and radio frequency mixers, and more particularly to an apparatus for mixing intermediate or radio frequency signals with local oscillator signals on a planar substrate. The invention further relates to star-type double-balanced mixer for performing very high radio frequency mixing in a uniplanar monolithic integrated circuit.
2. Related Technical Art
A variety of Radio Frequency (RF) mixers have been developed to facilitate combining a local reference oscillator output with either an intermediate frequency source or a radio frequency source for performing a desired up or down conversion. Mixers form the key link between RF antennas and processing or decoding circuits. In many applications, such as for advanced Monolithic Microwave Integrated Circuits (MMICs), the desire is to greatly reduce the size of mixers and related processing components to meet volumetric limitations otherwise addressed by MMIC elements, and further, to minimize power consumption.
One mixer configuration finding use in planar applications is a ring-type configuration Double Balanced Mixer (DBM) which allows more flexibility in component interconnection. An example of this type of mixer circuit is found in the article Broadband Double Balanced Mixer/Modulators by R. B. Mouse and S. M. Fukuchi, published in the Microwave Journal, pages 133-134, March 1969. Currently, most MMIC type DBMs utilize Field-Effect Transistors (FETS) and lumped-element baluns due to the difficulty in realizing a microwave balun to feed a planar star of Schottky diodes. Unfortunately, this type of mixer usually exhibits a narrow bandwidth due to elaborate matching requirements and is also relatively unstable due to temperature variations.
To create broadband mixing structures requires the use of active baluns having several FETs in a distributed configuration which is large in size and requires large DC power supplies. FET active baluns also introduce additional noise into MMIC double balanced mixers which degrades their performance.
Current planar designs for planar ring-type double-balanced mixers also require fabricating the Schottky diodes and other circuit elements on both sides of a support/base substrate. The assembly is then inserted into a waveguide channel for operation. This type of construction makes these mixers unsuitable for uniplanar or true monolithic integrated circuit applications.
Another type of double-balanced mixer useful in advanced RF applications is the star-type configuration double-balanced mixer, also discussed in the above-referenced article. There are currently no high performance passive uniplanar double-balanced mixers using Schottky diodes in a star configuration available for microwave or other very high frequency applications. There are currently no designs of this type of mixer which are readily useful for MMIC fabrication techniques.
What is needed is an apparatus or technique for manufacturing a uniplanar broadband star-type double-balanced RF mixer. It would be extremely advantageous if the mixer could be manufactured using known monolithic circuit fabrication techniques, such as employed in conventional MMIC applications. The mixer should provide low loss, MMIC compatible, operation.